The problem of obtaining various levels of brightness from a single lamp is well known.
In the case of incandescent lamps a reduction in brightness can be achieved simply by reducing the voltage applied to the filament. This, however, results in an appreciable reduction in the luminous efficiency, which is already appreciably low for this type of lamp (&lt;15%). In addition to this, the color characteristics of the light depend greatly on the applied voltage. Similar considerations also apply when the electrical power which the filament converts into radiating energy is varied from its rated power by other means. Many of the arrangements existing on the market also have stability problems associated with the use of potentiometers. In fact, with the passage of time the brightness of a lamp controlled by a potentiometer tends to vary, particularly when the moving terminal of the potentiometer lies in an intermediate position.
Multifilament lamps are known in the field of motor vehicle headlamps. In this field the various filaments are used alternately to obtain different lighting effects; for example, a lamp having a first filament for a dipped beam effect, a second filament for a full beam effect and a third filament for a fog-lamp effect is known from international patent application WO 8706325, incorporated herein by reference. These lamps also have a plurality of contacts--normally at least one at ground contact, and one for each filament.
In the case of fluorescent lamps, which are nevertheless of greater efficiency, more complicated electronic arrangements are used.
The problem of obtaining incandescent lamps with satisfactory light color characteristics is also well known.
It is also known from physics that the Stefan-Boltzman law, which links radiation to the fourth power of the absolute temperature, and Plank's law, which links radiation to frequency, and is bell-shaped, apply to so-called "black bodies." These laws apply to a lamp filament only to a first approximation. Of course the useful radiation from a filament is only that corresponding to the visible light band.